The Portevin-Le Chatelier Type for 316L(N) SS at Low Deformation Rate
I.M.W. Ekaputra
1,a*, Gunawan Dwi Haryadi
2,b, Rando Tungga Dewa
3,c, Budi Setyahandana
1,dand Sy Minh Tuan Hoang
4,e1Universitas Sanata Dharma, Paingan, Maguwoharjo, Depok, Sleman-Yogyakarta 55282, Indonesia
2Universitas Diponegoro, Jl. Prof. Sudharto, SH., Tembalang-Semarang 50275, Indonesia
3Universitas Pertahanan, Kawasan IPSC Sentul, Bogor-Jawa Barat, 16810, Indonesia
4Thu Dau Mot University, 6. Tran Van On. Phu Hoa Ward, Thu Dau Mot City, Binh Duong, 820000, Vietnam
a[email protected], b[email protected], c[email protected], d[email protected],
Keywords: Austenitic stainless steel 316L(N), Portevin Le-Chatelier, Work hardening
Abstract. This study determined the serrated yielding type for 316L(N) SS due to the Portevin-Le Chatelier effect under particular temperature conditions with the range of 24 to 655 oC at 10-5/s of plastic deformation rate. The 316L(N) SS was loaded by tensile test apparatus equipped with a three- zone furnace. The cylindrical specimen was put at the centre of the furnace. Since the test was conducted at various temperatures, a thermocouple was attached to the surface of the specimen. After the test, the engineering stress-strain curve was plotted, and the serrated yielding was observed. The results showed that the type A, B, D, and E were identified for a particular temperature. Type B was identified at the low-temperature region, and type A was identified at the high-temperature region. In addition, the work hardening rate curve was plotted to describe the plastic deformation characteristic.
Introduction
Strain rate sensitivity is one of the critical parameters that must be considered to determine the creep deformation mechanism [1]. The wide range of strain rates can lead to the change of flow stress. The three-stage in the creep curve occurred due to the alteration of strain rate value. An incremental change of strain rate occurred during the first stage, where the work hardening developed. A constant strain rate occurred at the second stage, where the recovery and hardening process was relatively balanced. The last stage is the rupture process, where the strain rate incrementally increase. It has been reported that at a particular strain rate value, the heterogeneous plastic deformation or also known as the Portevin Le-Chatelier effect, can lead to a loss in ductility and produce a rippled surface [2,3]. Macroscopically, the heterogeneous plastic deformation has been modelled by a well-known model, namely dynamic strain ageing (DSA). The DSA can be simply modelled when the solute atom experienced an ageing process [3-5]. The ageing process occurred since the mobile dislocation was impeded by an obstacle such as Forrest and pipe dislocation [4,5].
The serrated yielding phenomenon has been investigated for several metal alloys such as aluminum alloy, stainless steel, and nickel-based super-alloys [2,4-8]. The 316L(N) SS as one type of austenitic stainless steel is still an essential component for the power plant [5, 9-11]. This type of steel has several unique properties, exceptionally compatible with the liquid sodium coolant and stable at elevated temperatures. Several studies have been conducted to investigate the characteristic of 316L(N) SS under tension load for particular temperature and strain rate conditions [11].
Furthermore, it has been reported that uncommon due to serrated yielding was found for particular strain rate and temperature. Since this steel grade is still in use, more information about the serrated yielding for a wide range of temperature and strain rates is still required.
Key Engineering Materials Submitted: 2021-12-24
ISSN: 1662-9795, Vol. 939, pp 25-30 Accepted: 2022-08-11
doi:10.4028/p-6e556i Online: 2023-01-25
© 2023 Trans Tech Publications Ltd, Switzerland
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (#609446238-26/01/23,03:15:24)
In this study, the 316L(N) SS was investigated at elevated temperature and 10-5/s of deformation rate. The tension test was conducted in the temperature ranges of 24 oC - 655 oC. By plotting the engineering stress-strain curve, the serrated yielding was observed and identified. In addition, to investigate the strain rate effect on the plastic deformation process, the work hardening curve was plotted and discussed.
Experimental Procedure
In this study, the naming of “L(N)” is defined that this steel grade has a particular amount of Nitrogen (0.1 wt.%) and Carbon (< 0.03 wt.%). This 316L(N) SS has 0.020C–1.05Mn–0.0027P–0.005S–
17.1Cr–2.4Mo–12.2Ni–0.08N (wt.%). The ingot was normalized before being formed into a cylindrical form. Specimen size and dimensions followed the manufactured into a cylindrical specimen ASTM E8 [12]. The cylindrical specimen has a circular diameter of 6±0.01 mm and a gauge length of 30±0.01 mm. The specimen was intended to have a smaller diameter around the centre to produce a fracture at the centre position. All specimens were cut in the rolling direction and polished with #2000 of sandpaper grit. The specimen size and dimension were shown in Fig. 1.
The strain rate was controlled by setting the constant crosshead velocity of 10-5/s. Since the tensile test was conducted at elevated temperatures, the test followed the ASTM E21 [13]. The three-zone heating furnace was applied for the test at elevated temperature. The cylindrical specimen was put at the centre of the furnace and attached with a thermocouple on the specimen surface. Before loading the specimen, the furnace was heated until reaching the destination temperature with the range within
± 3 oC. A universal testing machine with a capacity of 50 kN was applied for the tensile test. Since no extensometer was not be used, the specimen elongation was measured based on the displacement of the crosshead movement.
Results and Discussion
The tensile test result for 316L(N) SS with the range of 24 to 655 oC at 10-5/s was represented by the engineering stress-strain curve in Fig. 2. It can be seen that the curve trend is decreasing with an increase in temperatures. The ultimate tensile strength and yield strength decrease significantly above room temperature. The recovery takes control with the higher temperatures. It is also seen that the curves are located at the narrow band for the temperature range of 190 oC - 655 oC. It has been reported that the narrow band indicated the serrated yielding occurrence [5]. The serrated yielding can also be seen at the non-linear curve (plastic area) in the engineering stress-strain curve where work hardening has occurred. From literature, the serrated yielding does typically not found at room temperature [5,11]. However, the serrated yielding is found for 316L(N) SS at 24 oC with 10-5/s of strain rate.
Fig. 1. Specimen size and dimesion for the tensile test.
Fig. 3 shows the magnification of the plastic curve for each temperature condition. It is seen that the different type of serrated yielding is formed. The type of serrated yielding is summarized in Table 1. Type B is observed at the range of 24 oC - 290 oC. Type B is indicated by the load oscillation about the centre of the stress-strain curve. Following the classical DSA theory, the oscillation occurred since
26 Applied Energy Research and Metallurgy
the ageing time of the solute atom was short [14]. Therefore, the mobile dislocation moved with high diffusivity. Type A is observed at the range of 335 oC - 440 oC. Type A is indicated by the sudden increase of load followed by load drop to the centre of the stress-strain curve. Type A is almost the same as type B, with the lower diffusivity of mobile dislocation due to the longer ageing time. The combination of type A+E is observed at the range of 510 oC - 545 oC. Type E typically appears at higher stress and higher temperature after type A. Type D is observed at 610 oC. Type D indicates the plateaus on the stress-strain curve. The combination of type A+B is observed at 655 oC.
Fig. 2. Engineering stress-strain curve of 316L(N) SS.
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
0 100 200 300 400 500 600 700 800
Eng.Stress (MPa)
Eng.Strain (mm/mm)
24 190 225 290 335 390 440 510 545 610 655
0.20 0.22 0.24 0.26 0.28 0.30 0.32
600 610 620 630 640 650
Stress, σ (MPa)
Strain, ε (mm/mm)
24
Type B serrations εa = 0.1846 σa = 588.85 MPa
0.25 0.26 0.27 0.28 0.29 0.30 0.31 0.32 516
518 520 522 524 526 528 530
Stress, σ (MPa)
Strain, ε (mm/mm)
190
TypeB serrations εb = 0.18047 σb = 483.88MPa
0.22 0.24 0.26 0.28
500 502 504 506 508 510 512 514 516 518
Stress, σ (MPa)
Strain, ε (mm/mm)
225
TypeB serrations εb = 0.26138 σb = 514.24MPa
0.27 0.28 0.29 0.30
504 505 506 507 508 509 510 511 512 513
Stress, σ (MPa)
Strain, ε (mm/mm)
290
Type B serrations εb = 0.25083 σb = 498.69 MPa
Key Engineering Materials Vol. 939 27
Fig. 3. Type of serration yield for each temperature condition.
Table 1. Identification of serrated yielding type for 316L(N)SS.
24 190 225 290 335 390 440 510 545 610 655
A - - - - O O O O O - O
B O O O O - - - - - - O
C - - - - - - - - - - -
D - - - - - - - - - O -
E - - - - - - - O O - -
The curve of work hardening rate of 316L(N) SS under various temperatures at 10-5/s is represented in Fig. 4. In this study, the work hardening rate for 316L(N) SS confirmed the previous study obtained by Samuel [15]. The work hardening rate,θ, is determined by calculating the derivative of true stress, σt, to the true strain, εt [14]. the three stages of work hardening rate was seen
0.22 0.24 0.26 0.28 0.30 0.32
472 476 480 484 488 492 496 500 504 508
Stress, σ (MPa)
Strain, ε (mm/mm)
335
Type A serrations εa = 0.13043 σa = 411.17 MPa
0.325 0.330 0.335 0.340
510 511 512 513 514
Stress, σ (MPa)
Strain, ε (mm/mm)
390
Type A serrations εa = 0.22503 σa = 471.54 MPa
0.22 0.24 0.26 0.28 0.30 0.32 0.34
480 485 490 495 500 505 510
Stress, σ (MPa)
Strain, ε (mm/mm)
440
Type A serrations εa = 0.24389 σa = 486.29 MPa
0.2 420 450 480
Stress, σ (MPa)
Strain, ε (mm/mm)
510
Type A serrations εa = 0.18033 σa = 440.43 MPa Type E serrations
εe = 0.27381 σe = 484.45 MPa
0.15 0.20 0.25 0.30
390 400 410 420 430 440 450 460 470 480
Stress, σ (MPa)
Strain, ε (mm/mm)
545
Type A serrations εa = 0.02641 σa = 220.58 MPa Type E serrations εe = 0.22749 σe = 440.85 MPa
0.02 0.03 0.04 0.05
200 210 220 230 240 250 260
Stress, σ (MPa)
Strain, ε (mm/mm)
610
Type D serrations εa = 0.0093 σa = 173.11 MPa
0.06 0.08 0.10 0.12
280 290 300 310 320 330 340 350 360 370
Stress, σ (MPa)
Strain, ε (mm/mm)
655
εa = 0.00446 σa = 161.84 MPa Type A serrations
Type B serrations εb = 0.01774 σb = 204.87 MPa
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Fig. 4. Work hardening rate for 316L(N) SS at 10-5/s.
by plotting θσt vs σt. The first stage was initially indicated by a rapid decrease in θσt followed by a gradual increase in θσt. The first stage is also called a transient stage, interpreted as plastic strain increases due to increasing mobile dislocation density after passing the elastic limit. The second stage was related to the hardening process where the slip was activated and resulted in heterogeneous dislocation. Finally, the third stage was identified as the recovery process.
Summary
In this preliminary study revealed that the serrated yielding type for 316L(N) SS was dominated by types A and B under various temperatures at 10-5/s. The remaining type of serrated yielding was also observed as type D and E. Following the engineering stress-strain curve, the work hardening rate showed that the recovery process was not found at room temperature. While above the room temperature, the recovery process occurred after the hardening process.
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0 100 200 300 400 500 600 700 800 900 10001100 0.00
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